Single electron devices

ABSTRACT

A single electron tunnelling device is formed by positioning between first and second electrodes a particle formed of a material having a first conductivity characteristic having a surface layer of a material of a second conductivity characteristic, the thickness of said layer being sufficiently small to support quantum mechanical tunnelling therethrough.

[0001] This invention relates to electronic components and, inparticular, to so-called single-electron devices and to methods ofmanufacture thereof.

[0002] The field of single electron devices emerged from investigationsof the tunnel junction, which consists of two electrodes of a conductingmaterial, separated by a thin layer of an insulating material having athickness of about one nanometre. According to the laws of quantummechanics, electrons have a small probability of tunnelling through suchan insulating layer. If a voltage is applied across the junction,electrons will tunnel preferentially in one particular direction throughthe insulator. Hence, they will carry an electric current through thejunction. The magnitude of the current depends on both the thickness ofthe insulating layer and the material properties of the conductingelectrodes.

[0003] In early 1985, Averin and Likharev attempted to predict thebehaviour of very small tunnel junctions with superconducting electrodesbut the equations were too complex to be easily solved. However, for asmall tunnel junction with electrodes of normal conductors, if aconstant electric current is passed through a junction, it will induce avoltage that oscillates periodically in time. These periodicoscillations have a frequency equal to the current divided by the chargeof an electron. This frequency is totally independent of any otherparameters of the system. An alternative view is that each oscillationrepresents the response of the device as a single electron tunnelsthrough the insulating layer. The phenomenon is known as single-electrontunnelling (SET) oscillations.

[0004] To understand this effect, one must appreciate how electriccharge moves through a normal conductor such as an aluminium wire. Anelectric current can flow through the conductor because some electronsare free to move through the lattice of atomic nuclei. Despite themotion of the electrons, any given volume of the conductor has virtuallyno net charge because the negative charge of the moving electrons isalways balanced by the positive charge of the atomic nuclei in eachsmall region of the conductor. Hence, the important quantity is not thecharge in any given volume but rather how much charge has been carriedthrough the wire. This quantity is designated as the “transferred”charge. This charge may take practically any value, even a fraction ofthe charge of a single electron. The reason for this is that charge isproportional to the sum of shifts of all the electrons with respect tothe lattice of atoms. Because the electrons in a conductor can beshifted as little or as much as desired, this sum can be changedcontinuously, and therefore so can the transferred charge.

[0005] If a normal conductor is interrupted by a tunnel junction,electric charge will move through the system by both a continuous and adiscrete process. As the transferred charge flows continuously throughthe conductor, it will accumulate on the surface of the electrodeagainst the insulating layer of the junction (the adjacent electrodewill have equal but opposite surface charge). This surface charge Q maybe represented as a slight continuous shift of the electrons near thesurface from their equilibrium positions. On the other hand, quantummechanics shows that the tunnelling can only change Q in a discrete way:when an electron tunnels through the insulating layer, the surfacecharge Q will change exactly by either +e or −e, depending on thedirection of tunnelling. The interplay between continuous charge flow inconductors and discrete transfer of charge through tunnel junctionsleads to several interesting effects. These phenomena can be observedwhen the tunnel junctions are very small and the ambient temperaturesare very low. (Low temperatures reduce thermal fluctuations that disturbthe motion of electrons.) In this case, if the charge Q at the junctionis greater than +e/2, an electron can tunnel through the junction in aparticular direction, subtracting e from Q. The electron does so becausethis process reduces the electrostatic energy of the system. (The energyincreases in proportion to the square of the charge and does not dependon the sign of the charge.) Likewise, if Q is less than −e/2, anelectron can tunnel through the junction in the opposite direction,adding e to Q, and thus again decrease the energy. But if Q is less than+e/2 and greater than −e/2, tunnelling in any direction would increasethe energy of the system. Thus, if the initial charge is within thisrange, tunnelling will not occur. This suppression of tunnelling isknown as the Coulomb blockade.

[0006] If the surface charge Q is zero initially, then the system iswithin the Coulomb blockade limits, and tunnelling is suppressed.Therefore, the current flowing from the source through wires will startto change the charge Q continuously. For convenience, assume that thedeposited charge rate is positive rather than negative. If the chargereaches and slightly exceeds +e/ 2, tunnelling becomes possible. Oneelectron will then cross the junction, making its charge slightlygreater than −e/2. Hence, the system is within the Coulomb blockaderange again, and tunnelling ceases to be possible. The current continuesto add positive charge to the junction at a constant rate, and Q growsuntil it exceeds +e/2 again. The repetition of this process produces thesingle-electron tunnelling (SET) oscillations: the voltage changesperiodically with a frequency equal to the current divided by thefundamental unit of charge, e:

[0007] To produce SET oscillations, tunnel junctions must be of a verysmall area and cooled to ensure that the thermal energy does notinfluence tunnelling. Typically, the device must be cooled totemperatures of about a tenth of a degree above absolute zero if thejunction is 100 nanometres in length and width.

[0008] European Patent Application EP 0 750 353 discloses a singleelectron tunnel device of this invention which includes a multipletunnel junction layer including multiple tunnel junctions; and first andsecond electrodes for applying a voltage to the multiple tunnel junctionlayer, wherein the multiple tunnel junction layer includes anelectrically insulating thin film and metal particles and/orsemiconductor particles dispersed in the electrically insulating thinfilm.

[0009] The electrically insulating thin film may be made of an oxide andthe particles may be of at least one type of metal selected from thegroup consisting of gold (Au), silver (Ag), copper (Cu), platinum (Pt),and palladium (Pd).of the particles. Their diameter may be 50 nm orless.

[0010] Fabrication of suitable structures to support single electrontunnelling has proved difficult. In particular, it has proved difficultto form films having the size and disposition which are suitable fortunnelling. However, we have now devised a method suitable for thefabrication of arrays of these devices.

[0011] According to one aspect of the present invention there isprovided a single electron tunnelling device comprising a particle of amaterial having a first conductivity characteristic having a surfacelayer of a material of a second conductivity characteristic, thethickness of said layer being sufficiently small to support quantummechanical tunnelling therethrough together with first and secondelectrodes positioned adjacent to said particle to facilitate the flowof current therebetween.

[0012] Said first and second electrodes may be superconducting.

[0013] In a preferred embodiment of the invention a plurality of suchparticles is positioned between said first and second electrodes.

[0014] There is also provided a method of fabricating single electrondevices comprising the steps of forming a plurality of particles forminga layer of a thickness sufficiently small to support quantum mechanicaltunnelling on the surface of said particles and positioning at least oneof said particles between a pair of electrodes to form a single electrondevice.

[0015] The invention will be particularly described by way of examplewith reference to the accompanying drawings, in which

[0016]FIG. 1 is a flow chart illustrating the method according to oneaspect of the present invention;

[0017]FIG. 2 is shows in schematic form apparatus for producingnanocrystals suitable for use for the fabrication of single-electrondevices;

[0018]FIG. 3 shows the structure of various nanoparticles;

[0019]FIG. 4 is a micrograph of a nanocrystal produced by the process ofaerotaxy

[0020]FIG. 5 is a schematic diagram of a prior art tunnel junction

[0021]FIG. 6 is a schematic diagram showing a barrier between ananoparticle and a conducting substrate;

[0022]FIG. 7 is a diagram illustrating the principle of movement of ananoparticle by means of an atomic force microscope; and

[0023]FIGS. 8a and 8 b are schematic diagrams illustrating theprinciples of device construction in accordance with the invention

[0024] Referring now to FIG. 1 of the drawings, this illustrates aprocess for controlled formation of simple and multi-layered metallicand semiconducting nanocrystals or nanoparticles suitable for singleelectron device fabrication. Ultra-fine particles of a Group III elementare formed as an aerosol. These are then filtered to select those of apredetermined size. A Group V precursor is then added and the mixtureprocessed to form nanocrystals of a III-V semiconductor.

[0025]FIG. 2 shows an aerosol production unit in accordance with aspecific embodiment of the invention. This comprises a furnace F1 whichgenerates metallic particles by sublimation. These particles are thencarried in a transport gas stream through a charger to a particle sizefilter DMA1 and thence to a second furnace F2 where the gas stream ismixed with the hydride of a Group V element and heated to formnanoparticles of a II-V semiconductor. The nanocrystals are thenfiltered to select those of a predetermined size which are thendeposited on to a substrate, which, preferably is a semiconductingwafer. in a deposition chamber DC. An electrometer E1 and pump Pu areconnectable to the flow line to create and measure the pressure therein.

[0026] In one embodiment, a semiconducting core nanocrystal is coated bya surface layer of a material with different properties, e.g. with alarger fundamental band-gap, fabricating nanocrystals, the compositionand size of which is tightly controlled. The approach is unusual in thatwe have managed to form, in an aerosol phase, metallic nanoparticles (ordroplets) having a narrow dimensional spread. The particles of elementsfrom the third column in the periodic table are later allowed to reactwith a vapour containing selected atoms or molecules from the fifthcolumn in the periodic table, resulting in the production ofnanocrystals of III-V semiconductors of uniform size. This controlrequires a completely saturated conversion of the primary metallicnanoparticle into the corresponding III-V nanocrystal.

[0027] Gallium arsenide nanocrystals, of approximate diameter 10 nm,have been produced and deposited on various substrates. The fabricationroute allows the production of nanocrystals with a very narrow sizedistribution. It utilises the formation of ultrafine gallium particlesand their self-limiting reaction with arsine at elevated temperatures.The kinetics of the reaction of gallium to produce gallium arsenidedepends on the temperature and the arsine flow. The temperature at whichthe reaction began was found to be as low as 200° C. This permitted theproduction of nanocrystals of compound semiconductors of predeterninedsize in a simple, reliable, and efficient way.

[0028] An important feature of a further embodiment of this invention isa new technique for controllable formation of a surface layer of adifferent semiconducting or insulating material on these originalnanocrystals. They may have a homogeneous core and a surface layer of asecond composition with an appropriate electronic structure for thesingle-electron device operation.

[0029] After a size selection, the semiconducting or metallicnanocrystal is exposed to a reacting gas environment while beingmaintained in the aerosol phase. In one embodiment, a mono-disperseaerosol of silicon nanocrystals is allowed to react with oxygen underclosely controlled conditions, leading to a controlled thickness of thesilicon particle being converted to silica. SiO₂ is an insulator withideal and well characterised interfaces with silicon. In a secondembodiment, mono-disperse nanocrystals of compound semiconductors, suchas indium arsenide, are allowed to interact with phosphorus-containinggaseous molecules, an interaction which results in exchange processes bywhich arsenic atoms in a finite depth surface layer are replaced byphosphorus atoms, hence transforming the surface to a surface layer ofIn(As)P. In a third embodiment, pre-fabricated nanoparticles of indiumreact with oxygen to form a skin of InO. In this embodiment, thesimplest single-electronic building block is formed by producing ahomogenous particle, exemplified by a spherical monodisperse particleshown in FIG. 3.

[0030] The second embodiment involves direct epitaxial deposition of adifferent material on the surface of a primary core, often calledhetero-epitaxy. The art of hetero-epitaxy on flat surfaces is at a veryadvanced stage but the use of nanoparticles as “substrates” foraerosol-phase epitaxial crystal growth is very novel. For theapplication of nanoparticles in single-electronics, however, this is ofgreat importance. Examples are the coating of a small band-gapsemiconductor with a thin epitaxial layer of a larger band-gap material,such as indium phosphide on the surface of indium arsenide or silicon onthe surface of a core of germanium. Finally there is a hetero-epitaxybased mechanism for formation of semiconductor particles surrounded byvery well controlled insulating layers, which can be achieved bysurrounding a nanocrystal of gallium arsenide (for example) with a fewmonolayers of epitaxially grown aluminium arsenide. At a late stage,this aluminium arsenide layer is allowed to react with oxygen to form alayer of aluminium oxide, most probably Al₂O₃, which is an excellentinsulator. Hence, the ideal hetero-expitaxial process will lend itselfindirectly to the formation of a few mono-layer-thick insulating layeron semiconductor particles. (FIG. 3)

[0031]FIG. 4 is a TEM image of an 8 nm indium phosphide particleproduced by the process of aerotaxy.

[0032] In the mechanism of single-electron devices, the most importantfundamental property is the existence of a central conductive islandwhich is coupled by tunnelling to source and drain electrodes andcoupled capacitively to a gate electrode. The size-related capacitanceof the central island should be sufficiently low that the electrostaticcharging energy E=e²/2C is much larger than kT and in an energy rangesuitable for device and circuit biasing. The dimensional requirementscan be described as:

[0033] for particle size, the diameter for room temperature operationshould be in the range 2-4 nm, corresponding to charging energy of a fewhundred meV, to be compared with kT (˜26 meV at room temperature).

[0034] for tunnelling gaps, the distances between conducting leads andconducting particle, and the distance between connected particles shouldsupport tunnelling, that is it should be in the range 1-3 nm.

[0035] In most prior demonstrations of single-electron phenomena, lowtemperatures at or below liquid helium boiling temperature (4K) havebeen employed. The tolerance for lithographic definition of the islandsize is much relaxed. In these studies, tunnelling distances are oftendefined by an aluminium film, which is converted by controlled oxidationinto an insulating thin film, placed in between the conductors.

[0036] Experiments performed at elevated temperatures, such as theboiling point of liquid nitrogen (77K) or at room-temperature (300K)have been performed with the use of small metallic (or semiconducting)particles but with the tunnelling distances controlled by a thininsulating film on which the particle rests and, for the secondelectrode spacing, by a tunnelling distance which is controlled by ascanning tunnelling microscope.

[0037] We have been able to fabricate planar single-electron deviceswhich are controllably created by a “nano-robot”, an atomic forcemicroscope (AFM), by manipulation of size-selected nanometre sizedparticles relative to pre-fabricated contacts. In this approachcapacitances are accurately controlled by the exact particle fabrication(by aerosol technique) and tunnelling gaps are governed by thecontrolled positioning of the nano-particles to create the propertunnelling current levels.

[0038]FIG. 5 illustrates a conventional thin film tunnel junctiondevice. The surface of a deposited film 11 is oxidise to form a thintunnel barrier 13 and a further conductor 15 is deposited thereon. Ananalogous device based on a small metallic particle is illustrated inFIG. 6. A thin oxide layer 17 is formed on a conductive substrate 19.and small metallic particle 21 is positioned thereon. Contact is made bymeans of the tip of a scanning tunnelling microscope 23. This principleis extended in the device illustrated in FIG. 7 in which a smallmetallic particle 25 is positioned between a source electrode 27 and adrain electrode 29 by means of an atomic force microscope.

[0039] A key feature of one aspect of the present invention is theprefabrication of particles in such a way that they provide theconducting core as well as tightly controlled tunnel-gap, building anetwork of identical capacitances and tunnelling rates permitsrandomness in lateral location within an ensemble of nano-particles.

[0040] The significance of the above aerosol-based fabrication ofgranular single-electron circuits is illustrated in FIGS. 8a and 8 bwhich show a two-dimensional arrangement of nano-particles P between twoelectrodes E1, E2 with non-identical (FIG.8a) vs identical cores (FIG.8b) as well as with random vs well-controlled tunnel barriers 31. Thetunnel barrier in most cases is exactly twice the shell thickness, intwo-dimensional as well as three-dimensional randomly arranged arrays.The key feature is that, due to nature of the single-electron tunnellingcharacteristic, for a macroscopic device the number of nanoparticles (ineither two or tlree dimensions) between the electrodes is not critical.

1. A single electron tunnelling device comprising a particle togetherwith first and second electrodes positioned adjacent to said particle tofacilitate the flow of current therebetween characterised in that saidparticle is formed of a material having a first conductivitycharacteristic having a surface layer of a material of a secondconductivity characteristic, the thickness of said layer beingsufficiently small to support quantum mechanical tunnellingtherethrough.
 2. A single electron tunnelling device according to claim1 characterised in that said device includes a plurality of saidparticles to define a current path between said first and secondelectrodes.
 3. A single electron tunnelling device according to claim 1characterised in that said material having said first conductivitycharacteristic is substantially homogenous.
 4. A single electrontunnelling device according to any one of claims 1 to 3 characterised inthat said surface layer is semiconducting.
 5. A single electrontunnelling device according to any one of claims 1 to 3 characterised inthat said surface layer is insulating.
 6. A single electron tunnellingdevice according to claim 1 characterised in that said surface layer isgallium arsenide.
 7. A single electron tunnelling device according toclaim 1 characterised in that said surface layer is indium oxide.
 8. Asingle electron tunnelling device according to claim 1 characterised inthat said surface layer is indium arsenide phosphide.
 9. A singleelectron tunnelling device according to claim 1 characterised in thatsaid surface layer is silica.
 10. A method of fabricating singleelectron devices comprising the steps of forming a plurality ofparticles forming a layer of a thickness sufficiently small to supportquantum mechanical tunnelling on the surface of said particles andpositioning at least one of said particles between a pair of electrodesto form a single electron device.
 11. A method of fabricating singleelectron devices according to claim 10 characterised in that a furtherstep of selecting particles of predetermined size takes place prior tothe step of forming said layer.
 12. A method of fabricating singleelectron devices according to claim 10 or 11 characterised in that saidplurality of particles is formed as an aerosol.
 13. A method offabricating single electron devices according to claim 10 or 11characterised in that said layer is formed by the chemical modificationof the surface of said particles.
 14. A method of fabricating singleelectron devices according to claim 10 or 11 characterised in that saidlayer is formed by the expitaxial deposition of a material on thesurface of said particles.
 15. A method of fabricating single electrondevices according to claim 10 characterised in that the positioning ofsaid particle is performed by means of an atomic force microscope.
 16. Amethod of fabricating a single electron device comprising the steps:forming a plurality of particles; forming on the surface of eachparticle a peripheral layer of a thickness sufficiently small to supportquantum mechanical tunnelling therethrough; providing a pair ofelectrodes and positioning at least one of said particles between saidpair of electrodes to form a single electron device.
 17. A method offabricating a single electron device according to claim 16, wherein afurther step of selecting particles of predetermined size takes placeprior to the step of forming said peripheral layer.
 18. A method offabricating a single electron device according to claim 16, wherein saidplurality of particles is formed as an aerosol.
 19. A method offabricating a single electron device according to claim 16, wherein saidperipheral layer is formed by chemical modification of the surface ofeach of said particles.
 20. A method of fabricating a single electrondevice according to claim 16, wherein said peripheral layer is formed bythe epitaxial deposition of a material on the surface of each of saidparticles.
 21. A method of fabricating a single electron deviceaccording to claim 16, wherein the positioning of said particle isperformed by means of an atomic force microscope.
 22. A method offorming a single electron tunnelling device comprising: forming aparticle of a material having a first conductivity characteristic,forming on the particle a semiconducting surface layer of a secondconductivity characteristic, the thickness of said layer beingsufficiently small to support quantum mechanical tunnellingtherethrough; and positioning said particle between first and secondelectrodes to provide a current path between the electrodes.
 23. Amethod of forming a single electron tunnelling device, comprising:forming a particle of a material having a first conductivitycharacteristic; forming on the surface of the particle a surface layerof gallium arsenide, the thickness of said layer being sufficientlysmall to support quantum mechanical tunnelling therethrough; andpositioning said particle between first and second electrodes to providea current path therebetween.
 24. A method of forming a single electrontunnelling device, comprising: forming a particle of a material having afirst conductivity characteristic; forming on the surface of theparticle a peripheral layer of indium oxide, the thickness of said layerbeing sufficiently small to support quantum mechanical tunnelling therethrough; positioning said particle between first and second electrodesto provide a current path therebetween.
 25. A method of forming a singleelectron tunnelling device, comprising: forming a particle of a materialhaving a first conductivity characteristic; forming on the surface ofthe particle a peripheral layer of indium arsenide phosphide, thethickness of said layer being sufficiently small to support quantummechanical tunnelling therethrough; positioning said particle betweenfirst and second electrodes to provide a current path therebetween. 26.A method of forming a single electron tunnelling device, comprising:forming a particle of a material having a first conductivitycharacteristic; forming on the surface of the particle a peripherallayer of silica, the thickness of said layer being sufficiently small tosupport quantum mechanical tunnelling therethrough; positioning saidparticle between first and second electrodes to provide a current paththerebetween.
 27. A method forming an electronic device, comprising:forming at least one nanoparticle having an inner core of a conductivematerial of predetermined size of nanometer dimensions; forming on theinner core, an outer shell of a controlled thickness of nanometerdimensions and of a further material which is different from that of theinner core; and providing first and second electrodes, and providing acurrent flow path therebetween comprising said at least one particle,the characteristics of current flow in the current flow path beingdetermined by electron tunnelling via said outer shell and inner core.28. A method according to claim 27, wherein said further material isinsulating.
 29. A method according to claim 28, wherein said furthermaterial is an oxide of one of: silicon, indium, aluminium.
 30. A methodaccording to claim 27, wherein said further material is semiconducting.31. A method according to claim 30, wherein said semiconducting materialcontains one of the following: indium, silicon.
 32. A method accordingto claim 27, wherein said conductive material contains one of: silicon,germanium, indium, gallium.
 33. A method according to claim 27,comprising providing a multiplicity of said nanoparticles stackedadjacent one another whereby to provide said current flow path.
 34. Amethod of forming a nanocrystal in the form of a particle that isdefined by a size of nanometer dimensions, the method comprising:forming, in an aerosol, a core particle of an electrically conductivematerial and having a size of predetermined nanometer dimensions; andforming epitaxially on the core particle, by the action of gas on theaerosol, an outer shell of a further material that is different fromthat of the core, and having a controlled thickness of nanometerdimensions.
 35. A method according to claim 34 wherein said conductivematerial contains one of: silicon, germanium, indium, gallium.
 36. Amethod according to claim 34, wherein said further material issemiconducting.
 37. A method according to claim 34, wherein saidconductive material contains one of the following: indium, germanium,gallium; and said further material is semiconducting and comprises oneof the following: indium, silicon, aluminium.
 38. A method of forming ananocrystal in the form of a particle that is defined by a size ofnanometer dimensions, the method comprising: forming in an aerosol, acore particle of an electrically conductive material and having a sizeof predetermined nanometer dimensions; and exposing the gas to anaerosol, the gas reacting with the surface of the core particle to forman outer shell of a further material that is different from the materialof the core particle, and that has a controlled thickness of nanometerdimensions.
 39. A method according to claim 38, wherein the gas reactswith the surface of the core particle to form an oxide of the corematerial.
 40. A method according to claim 39, wherein the core particlecontains one of: silicon, indium.
 41. A method according to claim 38,wherein the gas reacts with the surface of the core particle by anexchange process, wherein atoms in the surface of the core particle areexchanged for atoms in the gas.
 42. A method.according to claim 41,wherein the core material is a compound semiconductor, and the gascomprises molecules containing phosphorus.
 43. A method according toclaim 42, wherein the core material comprises indium arsenide, andarsenic atoms are replaced by phosphorus atoms.
 44. A method of forminga nanocrystal in the form of a particle that is defined by a size ofnanometer dimensions, the method comprising: forming from an aerosol, acore particle of an electrically conductive material and having a sizeof predetermined nanometer dimensions; and forming epitaxially on thecore, by the action of gas on the aerosol, an outer shell of a furthermaterial that is different from that of the core, and having acontrolled thickness of nanometer dimensions; and reacting the outershell to form an oxide of the further material.
 45. A method accordingto claim 44, wherein the conductive material is gallium arsenide, thefurther material is aluminium arsenide, and said oxide is aluminiumoxide.
 46. A method of forming nanocrystals comprising: a) forming anaerosol of particles of a predetermined conductive material anddiameters of nanometer dimensions; b) filtering said aerosol ofparticles to provide particles with a narrow predetermined spread ofdiameters; and c) processing the filtered aerosol with a vapour of amaterial in order to form a shell of a further material on each aerosolparticle and of a controlled thickness, said further material beingdifferent from said predetermined conductive material.
 47. A methodaccording to claim 46, wherein said processing in step c) comprisesforming by an epitaxial process said shell.
 48. A method according toclaim 47, wherein said conductive material contains one of: indium,germanium, gallium; and said further material includes one of:indium,silicon, aluminium.
 49. A method according to claim 46, includingthe further step of reacting the further material of the outer shell toform an oxide.
 50. A method according to claim 49, wherein said innercore comprises gallium arsenide, said further material comprisesaluminium arsenide, and said oxide comprises aluminium oxide.
 51. Amethod according to claim 46, wherein in said step c) said vapour reactswith the surface of the particle in order to form said shell bymodification of the surface of the particle.
 52. A method according toclaim 51, wherein the modification is an exchange process.
 53. A methodaccording to claim 51, wherein the particle material is a compoundsemiconductor, and the vapour comprises molecules containing phosphorus.54. A method according to claim 53, wherein the particle materialcomprises indium arsenide, and arsenic atoms are replaced by phosphorusatoms so that the material of the shell is indium arsenide phosphide.55. A method according to claim 51, wherein the modification isformation of an oxide.
 56. A method according to claim 55, wherein thematerial of the particle is one of: silicon, indium.
 57. A methodaccording to claim 46, wherein said step a) comprises: forming anaerosol of group III metallic particles, and filtering the aerosol toprovide particles with a narrow predetermined dimensional spread; andreacting the aerosol with a group V precursor gas, in order to providean aerosol of particles consiting og III-V semiconductor material.